Field of the Invention
The present disclosure relates to an electrophotographic photosensitive member, a process cartridge, an electrophotographic apparatus, and a phthalocyanine pigment.
Description of the Related Art
Phthalocyanine pigments are superior in function as a photoconductor and are used as a material for electrophotographic photosensitive members, solar cells, sensors, switching elements, and the like. It has been known that phthalocyanine pigments exhibit different electrical properties, depending on the crystal form thereof, and also on the manufacturing process (which is different among treating methods, such as UV irradiation, pulverization, and using solvent, or among the synthesizing methods) even if the crystal form is the same.
Phthalocyanine pigments (particularly titanyl phthalocyanine and gallium phthalocyanine pigments) are highly sensitive to light in a wide range of wavelengths and are, accordingly, used as the charge generating material of the electrophotographic photosensitive member of electrophotographic apparatuses using a semiconductor laser capable of oscillation in a wide range of wavelengths from short wavelengths to long wavelengths as an image exposure device.
Unfortunately, in electrophotographic photosensitive members using a highly sensitive phthalocyanine pigment, photo carriers (holes and electrons) are likely to be retained in the photosensitive layer (charge generating layer) due to the high sensitivity of the phthalocyanine pigment. The retained photo carriers cause an undesired phenomenon of ghosting in output images. Ghosting may appear as positive ghosting or negative ghosting. The positive ghosting is a phenomenon that forms a portion having a higher image density only in an area of the output image irradiated with light when the electrophotographic photosensitive member has previously been rotated, and the negative ghosting is a phenomenon that forms a portion having a lower image density in that area.
Some approaches have been studied to reduce ghosting. Japanese Patent Laid-Open No. 2006-72304 discloses an electrophotographic photosensitive member adopting a technique in which a mixture of a phthalocyanine pigment, an organic electron accepter, and a specific solvent is pulverized in a wet process so that the organic electron accepter is taken into the surfaces of the phthalocyanine pigment particles and/or the vicinities of the surfaces while the crystal form of the phthalocyanine pigment is changed. This document states that the use of the electrophotographic photosensitive member reduces ghosting more effectively. According to other studies (Japanese Patent Laid-Open Nos. 7-319188 and 2000-147811), the particle size and crystal lattice distortion of phthalocyanine pigments used in electrophotographic photosensitive members can be controlled by varying the conditions of pulverizing or dispersing the pigment.
Japanese Patent Laid-Open Nos. 7-319188 discloses an electrophotographic photosensitive member including a photosensitive layer containing a binder resin and a titanyl phthalocyanine pigment dispersed in the binder resin. The titanyl phthalocyanine pigment exhibits a CuKα X-ray diffraction spectrum having the strongest peak at a Bragg angle 2θ of 26.3°±0.2° with a half-width of 0.4° or less. The charged potential of this electrophotographic photosensitive member is not much reduced even by repeated use, and thus the electrophotographic photosensitive member exhibits good electrical properties. The half-width depends on the manufacturing conditions, such as the time for pulverization or dispersion, the size and specific gravity of the pulverization or dispersion media used, such as beads or balls, and the rotational speed of the pulverization or dispersion mill, such as a ball mill. This cited document explains that this is because the crystal lattice of the titanyl phthalocyanine can be irregularly distorted by the stress placed thereon by pulverization or dispersion.
Japanese Patent Laid-Open No. 2000-147811 discloses an electrophotographic photosensitive member including a photosensitive layer containing a binder resin and particles of phthalocyanine-based compound as a charge generating material, dispersed in the binder resin and having an average particle size in the range of 0.3 μm to 2 μm. According to this cited document, the electrophotographic photosensitive member can prevent a memory phenomenon and a dot defect (black spot) and has a withstand voltage that photosensitive members should have. This cited document explains that the reason why the memory phenomenon does not occur is that phthalocyanine-based compound particles having an average particle size of 0.3 μm or more do not have many interfaces with the binder resin and accordingly reduce the probability that electrons will be trapped. Also, the document explains the reason of high withstand voltage and no dot defect as below: the binder resin imparts a required withstand voltage to the photosensitive layer, because the charge generating material having an average particle size of 2.0 μm or less has a certain conductivity and accordingly loses the effect of reducing the effective thickness of the photosensitive layer, and also because such a charge generating material easily causes charges to be locally concentrated and thus loses the effect of increasing the probability of electrical breakdown.